1. Field of the Invention
The invention relates to a measuring container for biomagnetic measurements which can be used, in particular, for magnetocardiological measurements. Furthermore, the invention relates to a biomagnetic measuring system which comprises a measuring container according to the invention. Such measuring containers and biomagnetic measuring systems can be used, in particular, in the field of cardiology, but also in other medical fields, such as neurology, for example. Other applications are, however, also conceivable.
2. Description of the Background Art
In recent years and decades, magnetic measuring systems which have so far essentially been reserved for basic research have been moving into many fields of biological and medical sciences. In particular, neurology and cardiology are profiting from such biomagnetic measuring systems.
The basis of biomagnetic measuring systems is the fact that most cellular activities in the human or animal body are associated with electrical signals, in particular with electric currents. The measurement of these electrical signals themselves, which are caused by the cellular activity, is known, for example, from the field of electrocardiography. However, in addition to the purely electrical signals, the electric currents are also associated with a corresponding magnetic field whose measurement takes advantage of the various known biomagnetic measurement methods.
Whereas the electrical signals and their measurement outside the body are associated with various factors such as for example, the different electrical conductivities of the tissue types between the source and the body surface, magnetic signals penetrate these tissue regions virtually without interference. The measurement of these magnetic fields and their changes therefore enables conclusions relating to the currents flowing inside the tissue, for example electric currents inside the heart muscle. Thus, measurement of these magnetic fields with high temporal and/or spatial resolution over a particular region enables imaging methods which can, for example, reproduce a current situation of the various regions of a human heart. Other known applications lie, for example, in the field of neurology.
The measurement of magnetic fields of biological samples or patients, and/or the measurement of temporal changes in these magnetic fields constitutes a substantial challenge, however, in terms of measurement technology. Thus, for example, the changes in magnetic fields in the human body which are to be measured in magnetocardiography are approximately one million times weaker than the Earth's magnetic field. The detection of these changes therefore requires extremely sensitive magnetic sensors. In most cases, therefore, superconducting quantum interference devices (SQUIDs) are used in the field of biomagnetic measurements. As a rule, such sensors must typically be cooled to 4° K. (−269° C.) in order to reach or maintain the superconducting state, liquid helium normally being used to this end. The SQUIDs are therefore generally arranged individually or in a SQUID array in a so called Dewar vessel, and are appropriately cooled there. Alternatively, laser-pumped magneto-optical sensors are currently being developed which can exhibit approximately comparable sensitivity. In this case, as well, the sensors are generally arranged in an array in a container for the purpose of temperature stabilization.
The measurement of the extremely weak magnetic fields and/or their changes, which lie in the picotesla or subpicotesla range is naturally extremely sensitive to electromagnetic and magnetic disturbances. The magnetic field detectors of whatever type must be read out, a multiplicity of electronic devices being known for this purpose. However, this readout electronics reacts sensitively to parasitic external electromagnetic fields which can cause strong disturbances. Further disturbances result from the strong signal background of external magnetic fields such as, in particular, micropulsations of the earth's magnetic field or other magnetic fields, in particular temporally varying magnetic fields such as are brought about in multifarious ways in industrial society (for example by movement of large ferromagnetic masses such as trains, lorries etc., by way of example).
Various approaches are known from the prior art to the problem of solving disturbing influences. Thus, for example, WO 03/073117 A1 describes one of the many known devices for measuring magnetic fields in the subpicotesla range. The device uses a SQUID which is coupled inductively to an unscreened gradiometer. The device comprises a filter for filtering magnetically or electrically parasitic radio frequency interference. The aim is thus to lower the requirements on the electromagnetic screening of the measuring device and to enable a SQUID to be operated at all in rugged environments.
However, it has emerged in practice that despite an improved input filtering the signals of biomagnetic measurements can continue to be subject to strong influences from external electromagnetic and magnetic fields, since the abovementioned filters only facilitate the operation of the sensors, but generally have no effect on the electromagnetic disturbances in the frequency range of the biomagnetic signals to be measured. Consequently, it is impossible in practice in many cases to avoid provision of appropriate screening against the electromagnetic and/or magnetic fields. Thus, there have long been known from the civil (for example medical) and military fields eddy current screens against electromagnetic alternating fields which can be both of stationary and of movable configuration. As a rule, low frequency influences have been combatted by screens made from soft magnetic materials which have so far been predominantly of stationary installation.
EP 0 359 864 B1, which corresponds to U.S. Pat. No. 5,152,288, describes a device and a method for measuring weak spatially and temporally dependent magnetic fields. The device comprises a bearing device for holding the examination object, and a sensor arrangement with a SQUID array. Also described is a magnetic screening chamber which has a screening factor of at least 10 for magnetic alternating fields with a frequency of 0.5 Hz, a screening factor of at least 100 for magnetic alternating fields with a frequency of 5 Hz, and a screening factor of at least 1000 for magnetic alternating fields with a frequency of 50 Hz and above. Moreover the screening chamber has a screening factor of at least 1000 for high frequency alternating fields (frequencies greater than 10 kHz).
However, the screening chamber described in EP 0 359 864 B1 is extremely complicated in practice. In particular, there is a need for complicated structural measures in order to integrate the screening chamber into a building, since an appropriate pedestal must be provided which is of the order of magnitude of between 10 and 20 t and is produced from iron-free concrete. In practice, therefore, it is virtually ruled out to change the location of the device or to transport it.
A particular disadvantage of the device described in EP 0 359 864 B1 presides in the fact that despite the complicated screening numerous connections exist between the inner region of the screening chamber and the outer region, these being caused, for example, by leading the patient couch holder through the ground screen, and by further numerous posts led through the screening and by electrical bushings. These bushings have the effect, however, that magnetic and electromagnetic fields are coupled into the interior of the screening chamber and can sensitively impair measurement there.
A particular set of problems of the known screening chambers is, furthermore, in the arrangement of the required measurement electronics and/or of the computer systems required for the evaluation, in particular an image evaluation. If the measurement electronics and the computer systems are arranged entirely or partially in the interior of the screening chamber, they then disturb the measurements of electromagnetic fields generated by the electronics and/or computer systems. In addition, it is thereby impossible in practice in this case to operate the computer systems during the actual measurement, since the operating staff should not stay in the screening chamber during measurement, in order not to influence the measurement. If the measurement electronics and the computer system are, on the other hand, arranged outside the screening chamber, there is a need, in turn, for bushings via which electromagnetic and magnetic fields can be coupled into the interior of the screening chamber.
A further set of problems of such known screening chambers is that it is necessary to ensure that the patient is taken care of at all times, particularly in the field of magnetocardiography. In the case of patients with severe heart problems, in particular, it is necessary to ensure continuous monitoring as well as, in an emergency, also the application of immediate medical emergency measures such as, for example, a defibrillation. However, in the interior of a screening chamber—for example such as the screening chamber described in EP 0 359 864 B1, provision of such care is scarcely possible in practice since, for example, the application of a defibrillator would also simultaneously damage the sensor systems and/or the measurement electronics, and would thus occasion substantial costs. In addition, the space required for emergency medical care is lacking in most screening chambers.